Increasing demands for personal mobile communications equipment have motivated recent research activities to focus on the development of inexpensive, small size, low power consumption, and low noise level systems. To satisfy these requirements, one of the most important and indispensable circuit components is the on-chip silicon-based inductor.
As a result, miniaturization of the inductor on silicon has become a current key research area and extensive work has been done in this area. However, despite efforts by many researchers having skill in the art, achieving high performance on-chip inductors, i.e., high quality factor (Q), still remains a major problem especially when radio frequency integrated circuits (RFICs) are built on silicon.
In addition, high dynamic resistance of metal lines at GHz frequency ranges further degrades the inductor performance in CMOS technology as compared to those fabricated in monolithic microwave integrated circuits (MMICs).
Many fabricating techniques, processes, and materials have been proposed to improve the performance of on-chip inductors. Tedious processing techniques such as etching away the silicon substrate under the inductor have been introduced to remove the substrate parasitic effects completely. Despite achieving good results, industries are reluctant to adopt such a technique because of reliability issues such as packaging yield, as well as long-term mechanical stability.
The most critical factor hindering the performance of silicon-based inductors is the high resistive aluminum-copper (AlCu) interconnects used in silicon processes.
In comparison, thicker and less resistive gold (Au) metalization together with lossless substrate in gallium arsenide (GaAs) technology permits high performance inductors to be fabricated easily. To overcome high metalization resistance, a popular technique is to have the layers of metal stacked together, thereby achieving a high Q inductor.
Another possible alternative is to use an active inductor. In an active inductor high Q factor and inductance can be achieved in a really small silicon area. However, such an approach suffers from high power consumption and high noise levels that are not acceptable for low power and high frequency applications. In addition, performance of active inductors are very sensitive and dependent upon the inductor's biasing circuitry, making it time consuming and tedious to design.
A further possible alternative is to increase the thickness of metal because the Q value of an inductor is dependent upon its interior resistance (r) to the AC (alternate current) signal and increasing the thickness decreases the interior resistance. Unfortunately, the thickness of the metal cannot be increased without limit before encountering technological difficulties in lithography and etch.
As a result of the above, the simplest and most commonly used on-chip inductors are planar silicon-based spiral inductors, which require careful layout optimization techniques to improve performance.
Solutions to these problems have been long sought, but have long eluded those skilled in the art.